PURPOSE: To eliminate the need to set the lower limit of a source voltage which puts a load in stable operation to a value with some allowance, and to consume the energy of a power source effectively, by providing a constant output control circuit and a detecting circuit which outputs a discrimination signal.

CONSTITUTION: A reference voltage VREF1 is set to the boundary value between the collector-emitter voltage of a transistor(TR) Q1 in the active region and the collector-emitter voltage in the saturated region. When the load Z1 is driven, a TRQ4 is off and the discrimination signal S0 outputted from a comparator COM1 is at a low level when the TRQ1 operates in the active region, i.e. when a constant voltage is outputted to the load Z1 and at a high level when the TRQ1 operates in the saturated region, i.e. when a voltage lower than the specific constant voltage is outputted to the load Z1. When the load Z1 is not driven, the TRQ4 is on, so the discrimination signal S0 is at the low level all the time.